The present invention relates to "printed circuit" DC motors and more particularly to axial air gap brushless printed circuit DC motors.
Printed circuit DC motors as such are known. See, e.g., Swiggett U.S. Pat. No. 2,970,238. Brushless DC motors, as such, are also known. See, e.g., Ban U.S. Pat. No. 4,072,881 and Muller et al. U.S. Pat. No. 4,007,390.
As used herein the phrase "printed circuit" DC motor means a permanent magnet DC motor employing a low inductance armature formed, for example, by printed circuit techniques, by stamping or by winding. Baudot U.S. Pat. No. 3,144,574 discloses a low inductance armature formed by printed circuit techniques. Weiss et al. U.S. Pat. No. 3,566,727 discloses a low inductance armature formed from stampings. Keogh U.S. Pat. No. 3,550,645 discloses a low inductance wire wound armature.
In the present invention a flat, ironless, low inductance armature is bonded to the motor housing. The permanent magnets rotate with the rotor. In lieu of brushes the patented motor employs electronic commutation. Electro-optical sensors generate a digital representation of rotor position. The digital representation is decoded and, using brush substitution techniques, is used to selectively and progressively energize fields in the armature winding. All of this is accomplished while maintaining the high starting torque and variable speed capability of conventional DC motors.
The motor disclosed herein possesses many advantages. For example, it combines both high RPMs, e.g., on the order of 25,000, plus high horsepower, e.g., from 1/4 horsepower up to 5 or even 10 horsepower. Another advantage of the invention is increased thermal capacity resulting from bonding the armature to the motor mass.
Applications for the motor include the textile field, e.g., fiber drawing and sizing. There the requirements are for a motor which generates substantial horsepower at high speed and is designed to be continuously running. In this type of application synchronous reluctance motors have previously been employed. The synchronous reluctance motors, however, are sized for 1.5 horsepower in order to get up to speed quickly even though at steady state one 1/2 to 3/4 horsepower was needed. The DC brushless motor of the present invention has much better starting characteristics than the prior art synchronous reluctance motors.
In a DC brushless motor the position of the rotor magnets must be known at all times. In one embodiment of the present invention, which employs eight poles, three optical sensors are used to uniquely determine the positions of the magnets. The three optical sensors are employed in conjunction with an opaque disc having it circumference notched every forty-five degrees so as to generate a 3-bit gray code, i.e., one in which only one bit changes at any one time. This absolute coding permits the motor to be rotated in either direction when it is first energized. Other types of sensors, such as variable reluctance sensors may also be employed, so long as they provide absolute positioning information.
In the eight pole embodiment, switching occurs six times per quadrant or 24 times per revolution. At 25,000 RPM and switching 24 times per revolution, the switching frequency is 10 K Hertz. This high switching frequency is made possible by the low inductance of the armature.
In the eight pole embodiment, commutation may be effected by fields 45.degree. in width. Alternatively, the 45.degree. field may be broken into three 15.degree. fields. This reduces the torque ripple as a function of rotor angle to 5% or less.
The armature may be cut so as to produce several independent coils which may be reconnected as desired. In one configuration, this has the effect of doubling the K.sub.T of the motor while at the same time reducing the number of semiconductor switches by one-half.